feat: add subprocess session isolation for recipe execution

[bkrabach]

Summary

Adds an opt-in subprocess runner to amplifier-foundation that allows agent sessions to run in isolated child processes. When a subprocess exits, the OS reclaims ALL memory immediately — eliminating the RSS watermark accumulation that causes OOM kills during parallel recipe execution.

Problem

During parallel recipe execution (foreach with parallel: true), multiple agent sessions run concurrently in the same Python process. Each session loads ~300-400MB of modules. Python's allocator never returns freed pages to the OS after sessions complete. After multiple waves of parallel sessions (e.g., deep discovery running topdown + bottomup + combine), a single process grows to 35-37GB RSS and gets OOM-killed.

Solution

A foundation-layer utility (subprocess_runner.py) that:

• Serializes session config + prompt to a temp JSON file
• Spawns a child Python process via asyncio.create_subprocess_exec
• Child creates a fresh AmplifierSession, runs execute(), writes result to stdout, exits
• Parent reads result, OS reclaims all child process memory
• Includes max_subprocess semaphore (default 4) to prevent fork-bombing

Usage

from amplifier_foundation.subprocess_runner import run_session_in_subprocess

result = await run_session_in_subprocess(
    config=child_config,
    prompt=task,
    parent_id=parent_session.session_id,
    project_path=str(project_path),
    session_id=sub_session_id,
    timeout=1800,
)

Files

• New: amplifier_foundation/subprocess_runner.py (271 lines) — config serialization, child runner, __main__ entry point, parent spawner, concurrency semaphore
• New: tests/test_subprocess_runner.py (543 lines) — 21 tests covering round-trip serialization, child runner, parent spawner, error handling, timeout, concurrency
• Modified: amplifier_foundation/__init__.py — exports run_session_in_subprocess

Design

• Opt-in only — default behavior is unchanged (in-process spawning)
• No kernel changes — uses existing AmplifierSession API
• No new dependencies — stdlib only (asyncio, json, sys, tempfile)
• IPC: temp JSON file (parent→child) + stdout (child→parent) + stderr (errors)
• Companion PR for amplifier-app-cli wires this into session_spawner.py

Testing

21 tests covering:

• Config serialization round-trip (minimal, full, without session_id)
• Missing required keys and malformed JSON
• Child session runner (success, cleanup on error, no session_id)
• __main__ entry point validation
• Parent-side subprocess management (success, non-zero exit, timeout, temp file cleanup)
• Concurrency semaphore limiting

Design doc: amplifier-bundle-dot-graph/docs/plans/2026-03-21-subprocess-session-isolation-design.md